Environment variables in Linux CLI - Step-by-Step Execution

Concept Flow - Environment variables

Start shell session

↓

Set environment variable

↓

Use variable in commands

↓

Variable accessed by shell or programs

↓

End session or export variable for child processes

This flow shows how environment variables are set, used, and accessed in a shell session.

Execution Sample

Linux CLI

MY_VAR=hello
export MY_VAR
echo $MY_VAR

Sets an environment variable MY_VAR to 'hello', exports it, then prints its value.

Execution Table

Step	Command	Action	Variable State	Output
1	MY_VAR=hello	Assign MY_VAR locally	MY_VAR=hello (local)
2	export MY_VAR	Make MY_VAR available to child processes	MY_VAR=hello (exported)
3	echo $MY_VAR	Print value of MY_VAR	MY_VAR=hello (exported)	hello
4	exit	End shell session	Session ends

💡 Shell session ends, environment variables cleared unless saved in config files

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
MY_VAR	undefined	hello (local)	hello (exported)	hello (exported)	session ends

Key Moments - 2 Insights

Why does MY_VAR need to be exported to be available to child processes?

Because local variables are only visible in the current shell. Exporting MY_VAR (see step 2 in execution_table) makes it part of the environment passed to child processes.

What happens if you try to echo $MY_VAR before setting it?

The output will be empty because MY_VAR is undefined. This is shown by the variable state 'undefined' before step 1.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What is the output of 'echo $MY_VAR'?

A hello

B $MY_VAR

C undefined

D error

💡 Hint

Check the Output column at step 3 in the execution_table.

At which step does MY_VAR become available to child processes?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the Variable State column and the Action column in the execution_table.

If you remove the 'export' command, what will be the output of 'echo $MY_VAR' in a child process?

A hello

B empty string

C error message

D $MY_VAR

💡 Hint

Refer to the key_moments explanation about exporting variables.

Concept Snapshot

Environment variables store data for shell and programs.
Set with VAR=value, export with 'export VAR' to share with child processes.
Access with $VAR.
Local variables are not passed to child processes.
Exported variables are inherited by child processes.

Full Transcript

Environment variables are named values stored in the shell environment. You can set a variable by typing VAR=value. This variable is local to the current shell session. To make it available to programs or child shells, you must export it using 'export VAR'. Once exported, the variable is passed down to child processes. You can access the value by prefixing the variable name with a dollar sign, like $VAR. If you try to use a variable before setting it, it will be empty. When the shell session ends, the environment variables are cleared unless saved in configuration files.

Practice

1. What is the purpose of environment variables in a Linux shell?

easy

A. To speed up the internet connection

B. To store settings that programs can use during execution

C. To permanently delete files from the system

D. To create new user accounts automatically

Solution

1. Step 1: Understand environment variables

   Environment variables hold information like settings or preferences that programs can read while running.

2. Step 2: Identify the correct purpose

   Among the options, only storing settings matches the role of environment variables.

3. Final Answer:

   To store settings that programs can use during execution -> Option B

4. Quick Check:

   Environment variables = store settings [OK]

Hint: Remember: environment variables hold settings, not files or users [OK]

Common Mistakes:

• Confusing environment variables with file operations
• Thinking environment variables control hardware
• Assuming environment variables create users

2. Which command correctly sets an environment variable named MY_VAR with value hello for the current shell and its child processes?

easy

A. declare MY_VAR hello

B. set MY_VAR=hello

C. env MY_VAR hello

D. export MY_VAR=hello

Solution

1. Step 1: Recall how to set environment variables

   In Linux shells, export is used to set environment variables for the current shell and its children.

2. Step 2: Check each command

   set MY_VAR=hello is incorrect syntax; env MY_VAR hello does not set variable persistently; declare MY_VAR hello is invalid for environment variables.

3. Final Answer:

   export MY_VAR=hello -> Option D

4. Quick Check:

   Use export to set environment variables [OK]

Hint: Use 'export' to set environment variables for shell and children [OK]

Common Mistakes:

• Using 'set' instead of 'export' to set environment variables
• Trying to set variables without export, so children don't see them
• Confusing 'env' command usage

3. Given the commands below, what will be the output of echo $GREETING?

GREETING="Hello"
export GREETING
GREETING="Hi"
echo $GREETING

medium

A. Hello

B. $GREETING

C. Hi

D. Error: variable not found

Solution

1. Step 1: Understand variable assignment and export

   First, GREETING is set to "Hello" and exported, making it available to child processes. Then GREETING is reassigned to "Hi" in the current shell.

2. Step 2: Determine the value echoed

   The echo command prints the current value of GREETING, which is "Hi" after reassignment.

3. Final Answer:

   Hi -> Option C

4. Quick Check:

   Variable reassignment updates value = Hi [OK]

Hint: Last assignment before echo is the value printed [OK]

Common Mistakes:

• Thinking export freezes the variable value
• Expecting the first assigned value to print
• Confusing export with immediate output

4. What is wrong with this script snippet if the goal is to set PATH to include /usr/local/bin?

PATH=$PATH:/usr/local/bin
export PATH

medium

A. Nothing is wrong; it correctly appends to PATH

B. It should use export PATH=$PATH:/usr/local/bin in one line

C. It overwrites PATH instead of appending

D. It should prepend /usr/local/bin before $PATH

Solution

1. Step 1: Analyze the PATH modification

   The script appends :/usr/local/bin to the existing PATH variable, which is a common way to add directories.

2. Step 2: Check export usage

   Exporting PATH after assignment makes the new value available to child processes. This is correct usage.

3. Final Answer:

   Nothing is wrong; it correctly appends to PATH -> Option A

4. Quick Check:

   Appending and exporting PATH is correct [OK]

Hint: Appending PATH then exporting is standard practice [OK]

Common Mistakes:

• Thinking export must be on same line as assignment
• Confusing append with overwrite
• Assuming order of PATH parts always matters

5. You want to create a script that sets a temporary environment variable TEMP_VAR only for a single command execution, without affecting the shell's environment afterwards. Which command correctly does this?

hard

A. TEMP_VAR=value command

B. export TEMP_VAR=value; command

C. set TEMP_VAR=value command

D. command; export TEMP_VAR=value

Solution

1. Step 1: Understand temporary environment variables for one command

   Setting a variable before a command like TEMP_VAR=value command runs the command with that variable set only for its execution.

2. Step 2: Evaluate each option

   export TEMP_VAR=value; command exports variable globally; set TEMP_VAR=value command uses invalid syntax; command; export TEMP_VAR=value sets variable after command runs.

3. Final Answer:

   TEMP_VAR=value command -> Option A

4. Quick Check:

   Prefix variable before command for temporary env [OK]

Hint: Put variable before command to set it temporarily [OK]

Common Mistakes:

• Using export which affects shell permanently
• Trying to set variable after command runs
• Using incorrect syntax like 'set' for env variables